Solid Solution Ca Research Articles

Study of sites occupation and chemical environment of Eu 3+ in phosphate-silicates oxyapatites by luminescence

Silicate apatites called britholites have been studied due to their potential application as materials in the form of nuclear waste for the containment of actinides. The luminescence study of Eu 3+ in the solid solution Ca 10− x La x (SiO 4) y (PO 4) 6− y O z □ 2− z (with z=1+1/2( x− y)) allows us to predict the location of an eventual actinide ion and provide structural information on the expected behaviour of the structure towards irradiation damage. The luminescence study confirms the preferential location of the Eu 3+ ion in the 6 h site ( C s point symmetry) of the space group P6 3/m , where a strong crystal field due to Eu 3+–O(4) bond occurs. This bond competes with oxygen SiO 4 and PO 4 tetrahedra and the competing is found to be stronger when silicate groups are present. This is because silicate tetrahedra are less compact than phosphate tetrahedra and are able to approach Eu 3+ much closer. It is also shown that the crystal field strength decreases with silicate content. The luminescence spectra tend towards those which are more common of Eu 3+ luminescence in which phosphates are doped with rare earth ions. The spectra of luminescence versus temperature confirms that the crystal field between Eu 3+(6h)–O(4) is stronger when the temperature is low and the cell parameters are small.

Tie-lines and mixing properties of solid solutions in the system CaO-SrO-PbO-O at 1100 K

Phase relations in the system CaO-SrO-PbO-O2 at 1100 K have been determined by equilibrating samples with different compositions in air, oxygen, or evacuated ampoules for 7 days and characterizing quenched specimens by optical and scanning electron microscopy, energy-dispersive analysis of x-rays (EDX), and x-ray diffraction (XRD). There is a solid-state miscibility gap in the pseudo-binary system CaO-SrO, and continuous solid solubility between Ca2PbO4 and Sr2PbO4 at 1100 K. Substitution of Ca for Sr occurs only to a limited extent (∼2 mol.%) in SrPbO3. The calcium-rich solid solutions (Ca1−y Sr y )2PbO4 characterized by y≤0.255 are in equilibrium with PbO in air; compositions with y≥0.255 coexist with (Ca1−z Sr z )PbO3. There is a three-phase region involving the two monoxide solid solutions (Ca1−x Sr x )O on either side of the miscibility gap with x=0.24 and 0.71 and (Ca1−y Sr y )2PbO4 with y=0.96. Accurately determined are the locations of tie-lines between the solid solutions. Attainment of equilibrium was checked by the conventional tie-line rotation technique. The excess Gibbs energy of mixing of the solid solution with orthorhombic structure is obtained by an analysis of tie-line data; for the mixing of one mole of Ca and Sr represented by (Ca1−y Sr y )Pb0.5O2, ΔG E=y(1−y) [15,840−2950 y] J/mol. The thermodynamic properties suggest the onset of immiscibility in this solid solution below 884 (±5) K. The miscibility gap is asymmetric with a critical composition at y=0.43 (±0.02). Inside the triangle (Ca1−y Sr y )Pb0.5O2−(Ca1−z Sr z )PbO3−PbO, a small liquid-phase region is present close to the PbO corner, surrounded by three two-phase fields. Each corner of the approximately triangular liquid-phase region is associated with a three-phase field.

Crystal structural changes in titanite along the join TiO-AlF

We investigated the crystal structural changes in titanite solid-solution Ca(Ti,Al)(O,F)SiO 4 along the binary join TiO-AlF, on the basis of X-ray powder data and Rietveld refinement of seven synthetic titanites of intermediate compositions. Investigations with the transmission electron microscope allow us to narrow down the space group transition from P2 1 /a to A2/a to compositions between X Al = 0.09 and X Al = 0.18 [X Al = Al/(Al+Ti)]. The changes in most of the unit-cell dimensions along the binary join are non-linear, resulting in a small excess volume of mixing with a maximum at X Al = 0.54. The commonly observed trend of positive deviation of the excess volume of mixing near the large end-member, and negative deviation towards the small end-member seems to be reversed in this case. At AlF-contents larger than X Al = 0.6 the Ca-site and the O1-site in the titanite structure become increasingly over-bonded with Al-F substitution. At about X Al = 0.4 the octahedral cation-oxygen distances change significantly, indicating that the titanite structure undergoes a major atomic rearrangement at high AlF-contents in order to accommodate the increasingly different ionic size and charge. Generally, with increasing AlF content the polyhedra are being deformed rather than rotated. The changes in unit-cell dimensions, bond lengths and bond valence sums along the binary join suggest the presence of structural strain in AlF-rich titanite, especially at Al-F contents exceeding X Al = 0.4. The structural problems are obviously not significant enough to prevent the formation of Al-rich titanite in simple chemical systems as in our experiments. However, the structural strain may be significant enough to decrease the thermodynamic stability of Al-rich titanite in natural rocks compared to other Al- and F-bearing phases. This could partly explain the rare natural occurrence of titanite with X Al >0.54.

Synthesis, properties and crystal structures of α-Ca 3(BN 2) 2 and Ca 9+x(BN 2, CBN) 6—two compounds with BN 23− and CBN 4− anions

The isotypic compounds Ca 3(BN 2) 2 and Ca 9+0.5 x (BN 2) 6− x (CBN) x contain linear anions BN 2 3 − and BN 2 3 − together with isoelectronic linear CBN 4 − anions, respectively. Ca 3(BN 2) 2 is obtained in sealed niobium ampoules between 1480–1970 K from a stoichiometric mixture of Ca 3N 2 and BN and Ca 9+0.5 x (BN 2) 6− x (CBN) x from Ca 3N 2, BN, graphite and calcium. α-Ca 3(BN 2) 2 and Ca 9+0.5 x (BN 2) 6− x (CBN) x crystallize in the space group Im3̄ m with a x=0 =732.24(3) pm and a x = 2=739.82(8) pm. Compounds with a composition of Ca 3(BN 2) 2 and Ca 1 0(Bn 2) 4(CBN) 2 as well as solid solutions Ca 9+0.5 x (BN 2) 6− x (CBN) x with x=0.36, 1.43 were investigated. The compounds were characterized by X-ray structure analysis, DTA, IR-, Raman- and solid state NMR spectroscopic methods. A phase transition of α-Ca 3(BN 2) 2 to a low-temperature polymorph β-Ca 3(BN 2) 2 at 538 K was found.

Analytical electron microscopy of planar faults in SrO-doped CaTiO3

Oxide-rich planar faults within a perovskite matrix are the prevailing type of extended defects in polycrystalline SrO-doped CaTiO3. These defects form, depending on the temperature of sintering, random networks, or ordered structures. The chemistry of the polytypoid, the isolated planar faults, and the perovskite phase have been studied by spatially resolved electron energy-loss and energy-dispersive x-ray spectroscopies using a dedicated scanning transmission electron microscope. We have found that Sr ions from SrO additions preferably substitute Ca in the CaTiO3 lattice, thus forming a solid solution (Ca1–xSrx)TiO3. The surplus of Ca ions forms single and ordered CaO-rich planar faults in the host (Ca1–xSrx)TiO3 phase. Whereas the excess Ca segregates in a form of single planar faults at lower temperatures, it forms a stable polytypoidic phase at higher temperatures. For materials having up to 25 mol% of SrO additions, this phase has (Ca1–xSrx)4Ti3O10 composition, comprising a sequence of CaO faults followed by three (Ca1–xSrx)TiO3 perovskite layers. Analytical electron microscopy revealed that the composition of the single planar faults, formed at lower temperatures, is identical to that of polytypoids, which are stable at higher sintering temperatures.

The new phosphates Ca 9[formula omitted](PO 4) 7 ( M = Li, Na; Ln = rare earth, Y, Bi)

The new family of triple phosphates Ca 9MLn 2 3 (PO 4) 7 ( Ln = rare earth, Y, Bi; M = Li +, Na + has been synthesized and studied by X-ray powder diffraction, IR-spectroscopy, differential thermal analyses and luminescence. All compounds have the whitlockite-like structure (space group R3c). Cell parameters, IR and emission (Eu 3+) spectra are given. The luminescence properties of trivalent europium are analyzed in Ca 9NaEu 2 3 (PO 4) 7 and Ca 9LiEu 2 3 (PO 4) 7 at 290 K. The fluorescence lifetime of the 4F 3 2 − 4I 11 2 transition of Nd 3+ is reported for the solid solutions Ca 9Nd xGd 1 − x(PO 4) 7, Ca 3 − xNd 2x 3 (PO 4) 2 and Ca 9M(Nd xGd 1 − x) 2 3 (PO 4) 7 (M = Li, Na).

Simultaneous Occurrence of Sn 4+ on the Three Cationic Sites of the Garnet Structure: The Solid Solutions Ca xSn xGa 8-2 xO 12 (2.5 < x P;lt; 3.0)

The first example of the simultaneous occurrence of the same cation, namely Sn 4+, on the three cationic sites of the garnet structure is reported; the single phase solid solution Ca x Sn x Ga 8-2 x O 12, resulting from the double substitution 2Ga 3+ → Sn 4+ + Ca 2+ exists within the composition range 2.65 ≤ x ≤ 2.95, i.e., between the limiting compositions Ca 2.65Sn 2.65Ga 2.70O 12 and Ca 2.95Sn 2.95Ga 2.10O 12. Such compositions imply that the Sn 4+ cations are distributed over the three cationic sites of the garnet structure, e.g. for x = 2.65 (Ca 2.65Sn 0.35) VIII(Sn 2) VI(Ga 2.70Sn 0.30) IV. This has been definitely confirmed by X-ray powder diffraction structure calculations. In the Raman spectrum, the totally symmetric ( v 1) stretches of SnO 4 and GaO 4 tetrahedra are observed near 780 and 750 cm -1, respectively, whereas in the infrared spectrum, the corresponding antisymmetric vibrations coalesce in a broad, asymmetric absorption.

Preparation, Crystal Structure, and Properties of a New Double Metal Nitride, SrNiN, and of Ca 1- xSr xNiN (0 ≤ x ≤ 0.5) Solid Solutions

A new double metal nitride, SrNiN, and solid solutions Ca 1- x Sr x NiN (0 ≤ x ≤ 0.5) were prepared by the reaction of Sr 2N and/or α-Ca 3N 2 with nickel fine powder in a nitrogen atmosphere. SrNiN had an orthorhombic crystal lattice, Pnma, with a = 9.0859 Å, and b = 13.231 Å, and c = 5.2473 Å and was isostructural with BaNiN. The present products have crystal structures composed of infinite [NiN 2/2] 2- chains linked with tetrahedrally coordinated Sr 2+ and/or Ca 2+. The chain shapes are zigzag in SrNiN and linear in Ca 1- x Sr x NiN (0 ≤ x ≤ 0.5). Both crystal lattices have fcc nitrogen packing. The difference in chain shape can be understood in terms of a limited range of distortion in the tetrahedron supporting [NiN 2/2] 2- chains. All of these products were metallic conductors with residual electrical resistivities of order 10 -4 Ω cm at 10 K. Electrical resistivities at room temperature were 6.0 × 10 -3 Ω cm in SrNiN and 2-6 × 10 -4 Ω cm in Ca 1- x Sr x NiN. They showed temperature-independent magnetic behavior at temperatures down to 77 K.

Structural study and conductivity properties of Ca1−xNa2xTi4(PO4)6 solid solution

A complete solid solution Ca1−xNa2xTi4(PO4)6 was prepared between CaTi4(PO4)6 and Na2Ti 4(PO4)6 belonging to the NASICON structural family. The space group evolves from R3 for x⪕0.5, to R3c for x>0.5. For each composition, conductivity values obey the Arrhenius law. From the two end members, cationic substitution improves the conductivity which reaches a maximum for x⋍0.5. The structure of CaTi4(PO4)6 was refined in the R3 space group applying the Rietveld method to the X- ray powder spectra. The Ca2+ cations and vacancies are ordered along the hexagonal c direction. A correlation is proposed between conductivity results and structural features.

Structural Study of the Solid Solutions in a CaSi 2-LaSi 2 System

The crystal structure of the solid solutions in a CaSi 2-LaSi 2 system was studied by the X-ray Rietveld analysis. The layered structure of CaSi 2 was changed into the three-dimensional α-ThSi 2-type structure in the solid solutions Ca 1- x La x Si 2 (0.2 ≤ x ≤ 1.0). The lattice parameters of the solid solutions did not obey Vegard's rule. The structural analysis revealed that the Si-Si distances and the Si/ Si\\ Si bond angle of the three-coordinated coplanar group of the Si sublattice changed rather irregularly with the composition of the solid solutions. The Raman band due to the Si-Si stretching vibration was hardly influenced by the composition of the solid solutions. Magnetic susceptibility measurements suggested that the superconducting transition of the solid solution occurred at a temperature between 1.5 and 2.5 K.

The crystallography of CA 1−xF x using X-ray powder diffraction techniques

The system CA-CF shows two solid solutions series: C(A,F) and C(F,A). We have analysed the variation of the cell parameters of the solid solution CA 1−xF x (x indicating molar fraction of CF) depending on the initial weight percent of CF, using X-ray diffraction (XRD) and two different methods of cell parameter refinement: Peak Maxima-Matching Method and Rietveld Profile-Fitting Method. The solid solution end is placed at 16'5 wt% of CF (x=0.125). This value has been calculated through the aluminium population factor in the CA phase structure, as well.

Cation Distribution by Powder X-Ray Diffraction in Uvarovite-Grossularite Garnets Solid Solutions Synthesized by the Sol-Gel Method

Nominal compositions of uvarovite-grossularite solid-solutions Ca 3(Cr 1- x Al x ) 2Si 3O 12, series x = 0 to x = 1, have been prepared by sol-gel method and fired up to 1000°C. Five of these structures ( x = 0 to 0.5) have been refined using the Rietveld method from X-ray powder diffraction data taken with a Siemens D-500 diffractometer. The limit of solid solutions of these structures has been stabilized in 20% mole grossularite. The unit cell is cubic, Ia3d (230), Z = 8, a = 12.0205(5) to 11.9903(18) for the five structures presented.

Solid Solutions Ca 3Sn 2+ xSi(Ge) 1- xGa 2O 12 (0 ≤ x ≤ 0.95) and Tetrahedral Coordination of Sn 4+ in the Garnet Structure

Pure solid solutions Ca 3Sn 2+ x Si(Ge) 1-xGa 2O 12 with the garnet structure have been evidenced in the composition range 0 ≤ x ≤ 0.95. This implies that a part of the Sn 4+ cations (equal to x) is located on the tetrahedral sites, a quite unusual coordination for this cation in oxygen compounds. This has been definitely confirmed by X-ray powder diffraction structure calculations. Information about the cation-oxygen distances and the distortion of the different coordinated groups is presented. No band characteristic of the SnO 4 tetrahedra is observed in the IR spectrum, probably because of a strong mixing between SnO 4 and GaO 4 vibrations. In contrast, the totally symmetric stretching mode v 1 of the SnO 4 tetrahedra is easily observed near 775 cm -1 in the Raman spectrum, where it is well separated from the corresponding mode of the GaO 4 tetrahedra. The different vibrational behavior observed in IR and Raman spectra is discussed on the basis of the symmetry properties of the vibrations.

Investigation of the CaOMnO system

A series of solid solutions Ca x Mn 1− x O was studied thermodynamically in the range from 1000 to 1200 K by solid-state galvanic cells involving β″-alumina electrolytes conducting calcium and manganese ions, respectively. The experimental procedure allowed us to determine the CaO and MnO activities directly from the emf measurements. The solutions exhibited strong positive deviation from ideal behavior. They could be regarded as regular solutions.

Yttrium Substitution in CaS

Substitution of Y3+ in CaS results in the cubic rock-salt-type solid-solution Ca1−xY2x/3□x/3S, with x 0.37 and where □ denotes cation vacancies that form for charge compensation. The substitution range determined from polycrystalline samples prepared at 1025°C corresponds to a solubility limit of 16 mol% Y2S3 in CaS. Evolution of the unit cell edge with x indicates that the CaS lattice expands about the cation vacancies. Single-crystal X-ray diffraction studies of samples grown from a eutectic CaCl2–KCl flux revealed no indication of superstructure formation, indicating that Ca2+, Y3+, and vacancies are disordered across the cation sites. Thermogravimetric analysis curves for Ca0.7Y0.2S, CaS, and the ternary compound CaY2S4 indicate oxidative stability up to 430°, 410°, and 490°C, respectively.

Processus de clustérization au sein des solutions solides Ca 1−xLn x F 2+x (Ln=La, Nd, Gd) comportant un substituant trivalent lanthanidique de grande taille

The model of the processes of clusterization proposed for the anion-excess CaF 2-type solid solutions M 1−x 2+M′ x 2+αF 2+α x (α=1, 2, 3) has been applied to the solid solutions Ca 1− x Ln x F 2+ x (Ln=La, Nd, Gd) involving rare earth substitutional cations of a large size. The formation of 1:0:3 clusters when x increases in the composition domain (0.02⩽ x⩽0.42) has been confirmed. Two sublattices of vacancies located inside the clusters ( y 1) and in the immediate neighbourhood ( y 2) are related respectively to the F″ and F‴ interstitial fluoride ions. The Ca 1− x Gd x F 2+ x phase characterized by the existence of the 1:0:3 clusters and the presence of the smallest substitute, has, for a given value of x, the highest percentage of vacancies y 2 and the weakest space constraints. It results in the best electrical performance for that solid solution.

Defect clustering in double doped fluorite crystals with Lu and Gd

Abstract Solid solutions Ca1 x-yLuxGdy F2+x+y for 10−4 ≤ x ≤ 2 × 10−2 and y=0.0001 have been studied by electron paramagnetic resonance (EPR) and ionic thermal currents (ITC). It has been found that the ITC spectrum from 77 to 420 K is very weak and the main peak is attributed to the relaxation of both Lu3+-F− x and Gd3+F− i nn dipoles. No polarizable clusters are present in the temperature range explored here. The EPR spectra show the presence of Gd3+ tetragonal and cubic centers due to the local and non local compensation, respectively. The continuous decrease in the molar fraction of Gd3+ tetragonal centers together with the low concentration of Lu nn dipoles is an evidence of the existence at these low and intermediate concentrations of large clusters such as the cubo-octahedral hexamer which has been proposed for CaF2 crystals very highly doped with small trivalent cations.

Etude des proprietes de conduction ionique des solutions solides Ca 1− xTh xF 2+2 x et Ca 1− xU xF 2+2 x

A comparative study of the ionic conductivity properties of the solid solutions Ca 1− x Th x F 2+2 x (0 ≤ x ≤ 0.18) and Ca 1− x U x F 2+2 x (0 ≤ x ≤ 0.19) has been undertaken and correlations between the electrical properties and the short-range order within these phases have been established. The model of the processes of clustering proposed for the anion excess CaF 2-type solid solutions M 2+ 1− x M′ 2+α x F 2+α x (α = 1, 2, 3) has been applied to the solid solutions Ca 1− x M″ x F 2+2 x ( M″ =Th, U). The formation of the 1:0:3 clusters is confirmed when x increases in the composition domain (0.01 ≲ x ≤ x L) ( x L: upper limit of substitution rate). Two sublattices of vacancies are located inside the 1:0:3 clusters (y 1) and in the immediate neighborhood (y 2); these are related respectively to the F″ and F‴ interstitial fluoride anions. The Ca 1− x U x F 2+2 x phase, involving the smallest tetravalent cation, has a larger number of vacancies y 2 and weaker space constraints for the same substitution rate. It results in better electrical properties for that solid solution than for Ca 1− x Th x F 2+2 x . The solid solutions Ca 1− x M″ x F 2+2 x ( M″ =Th, U) are characterized by the same process of clustering as the solid solutions Ca 1− x Ln x F 2+ x ( Ln =La, Nd, Gd) involving rare earth substitutional cations of large size.

19F NMR study of the fluorite‐Type Ca1−xThxF2+2x solid solution

AbstractAn investigation is made by 19F NMR of the disordered fluorite‐related solid solution Ca1−xThxF2+2x (0 ≦ x ≦ 0.18). The existence is shown of different fluoride sublattices at low temperatures and the setting‐up of partial exchanges between them at increasing temperature. The identity of these sublattices with those identified by neutron diffraction is discussed. The NMR study allows to confirm the formation of clusters of 1:0:3 type in Ca1−xThxF2+2x. The results obtained are so in good agreement with the composition dependence of the ionic conductivity properties and are satisfying with respect to the model of the processes of clustering proposed for the anion excess CaF2‐type solid solutions: indeed, within Ca1−xThxF2+2x, the F‴ interstitial fluoride ions seem to play the main role in the long‐range motions.

Corrélations entre propriétés électriques et ordre à courte distance au sein des solutions solides Ca 1− xY xF 2+ x et Ca 1− xLu xF 2+ x

A model has been proposed to correlate the composition dependence of electrical properties and the progressive cluster growth with increasing x in a fluoride anion excess CaF 2-type solid-solution of M 2+ 1− x M′ 2+ α x F 2+ αx ( α = 1,2,3). According to the model, the density of interstitial fluorine ions and the density of vacancies at normal sites are respectively represented by the functions y int. and y □ which depend on three parameters called λ, m, and x e: Y int. = (λ−α)mx 3+λ(m−α)x 2 e (λ−α)x 2+(m−α)x 2 e Y □ = (λ−α)(m−α)(x 3+x 2 e x (λ−α)x 2+(m−α)x 2 e The λ and m parameters respectively define the clustering conditions for the lowest and highest values of x. The x e parameter is the degree of substitution for which a maximum of conductivity in the composition dependence of the electrical properties is encountered. This model, applied to the solid-solutions Ca 1− x Y x F 2+ x (0 ⩽ x ⩽ 0.38) and Ca 1− x Lu x F 2+ x (0 ⩽ x ⩽ 0.40), shows the progressive transformation of 4:4:3 monomer clusters into cubooctahedral 8:12:1 clusters when x increases. The percentage of cubooctahedral clusters in these solid-solutions quenched from 950°C has been determined as a function of composition.